There's an issue with language filter on Categories and Items page (maybe somewhere else as well), which hasn't been fixed for quite a while already (had it in 2.6 or something). Due to this issue categories or items list is always empty. Problem's with this line:

$language = $application->getUserStateFromRequest($option.$view.'language', 'language', '', 'string');

It always returns "en" for me, however, language filter isn't even used. No matter what I select, it's always "en" and list is empty due to this. I have to edit models for categories and items after every upgrade.

Hello,

Are you referring to the admin area?

Yep, exactly.

Can you try installing the latest DEV version?
github.com/getk2/k2

Yep, all good now. I looked in the code and this is exactly what was needed. Thank you.

You 're welcome :)